1. Field of the Invention
The invention relates in general to tachometers, and more specifically to tachometers for developing speed indications from pulses produced at a rate proportional to speed.
2. Description of the Prior Art
Certain applications, such as elevator systems, utilize one or more tachometers for providing monitoring and control systems for properly operating the elevator system. For example, U.S. Pat. No. 4,085,823, which is assigned to the same assignee as the present application, utilizes two tachogenerators to develop DC voltages representative of speed. One tachogenerator measures the rotational speed of the elevator drive motor, and the other tachogenerator measures the rotational speed of the governor sheave, which is driven by wire ropes attached to the elevator car. The outputs of the two tachogenerators, among other things, are used in a self-checking manner wherein they alternately develop speed points when the elevator system is operating correctly. The speed points and car position are also continuously compared to detect the need for an auxiliary terminal slowdown speed pattern, and also to detect the need for an emergency terminal stop. While one highly accurate, low noise, low ripple DC signal proportional to drive motor speed is required for speed and acceleration feedback signals in the motor control loop, the tachogenerator which monitors the governor sheave speed need not provide a smoothly changing signal. The latter is used in reference circuits which compare signal levels to develop speed checking points. Thus, it would be desirable to provide a less costly tachometer for measuring the rotational speed of the governor sheave, which is proportional to car speed, if the new tachometer can provide the resolution and accuracy required, especially towards the lower end of the speed range, and if the new tachometer can be economically constructed.